The present invention relates to process enhancements in pickling of refractory metals (Ta, Nb, Ti, Mo, W, V, Cr, Zr, Hf and alloys and mixtures), and more particularly Ta and Nb, incident to cleaning mill products and/or fabricated parts at the conclusion of mill processing or end product fabrication and at the end of intermediate steps, e.g. pickling drawn Ta wire or strip after each drawing or rolling pass, or each series of passes in a larger sequence, to remove lubricant and surface contaminants prior to an intermediate anneal. The term pickling is used herein in a broadest sense of cleaning the surface of a metal by a strong etching solution. Small, but valuable, portions of the metal are necessarily removed from the surface of the metal product and have to be recovered.
It is known that conventional pickling baths for Ta which include HF/HNO3 solutions are vulnerable to nitrous oxide emissions and associated hazards to the environment and also leading to instability of the bath itself. It is also known that nitrous oxide emissions from HNO3-containing pickling baths can be reduced by controlled hydrogen peroxide addition. German patent application A-25-32773; U.S. Pat. No. 4,938,838; and Japanese patent application 50-110682. The mechanisms of HF/H2O2 interaction are discussed in an article of Chakravorti et al. xe2x80x9cFirst Electrosynthesis of Transition Metal Peroxofluoro Complexes . . . xe2x80x9d at 12 (6) Polyhedron 683-87 (1993) (concerning synthesis of such complexes in dissolution of powders of Ta, Nb, V, Mo, W) and in the textbook, Cotton et al, Advanced Inorganic Chemistry 791-92 (5th Ed""n Wiley). However, a practical in-line pickling process for pickling with substantially reduced nitrous oxide emissions remains elusive.
It is an object of this invention to provide such a process.
We have determined that wire and strip of Ta and other refractory metals can be effectively pickled at high rates to remove contaminants located at the metal surface as well as lubricant residues, without excessive loss of metal, by a pickling bath comprising an aqueous solution of HF/H2O2 with the HF and H2O2 in a weight ratio (gpl/gpl) of from 4:1 to 15:1, while limiting the absolute amounts of HF to under 800 gpl, preferably under 400 gpl and more preferably about 10-200 gpl H2O2 and to under 200 gpl and more preferably about 20-50 in relation to reasonably sized batches of tantalum (and correspondingly reduced as to other more etchable metals) to avoid excessive metal removal. The conditions are adjusted to achieve the effective surface etching of metal in 5-20 minutes. Temperature of the bath should be maintained between 20 and 40xc2x0 C. for a median bath composition of 200 gpl HF and 10-50 gpl H2O2.
Commercially available H2O2 solutions include stabilizers that may be provided in amounts of about 50-100% of peroxide amounts. Build up of Ta in solution to upper limits must be resolved by a bath regeneration step that includes Ta recovery. Addition of potassium fluoride (KF) provides a means of such regeneration/recovery as is indicated in the Apr. 23, 1982 Russian Inventor""s Certificate of Balyasov et al. entitled xe2x80x9cA Solution for the Chemical Pickling of Niobium and Its Alloysxe2x80x9d. KF is added initially in small amounts to precipitate K2TaF7 crystals and limit heat due to exothermic reaction, then in larger amounts and with cooling to accelerate crystallization, the KF amounts being about 1.0 to 1.5 times stoichiometric in relation to estimated Ta content in solution to limit excessive KF residuals in the bath after regeneration.
The bath composition can accept a loading of etched metal up to and in some cases exceeding 300 gpl than allowing long usage before metal disposal and regeneration. Dissolved tantalum can be processed to a salt form (K2TaF7)reusable as a tantalum source by reduction in a manner well known in the art.
Other objects, features and advantages will be apparent from the following detailed description of preferred embodiments taken in conjunction with the accompanying drawings in which: